I would go 3", which is a little more than whats in the pole base.
When you go to re-install it, make sure you cut notches on the short sides so it will compress, if you notice when you take it apart the factory did it.
Everyone complains about how hard it is to fit the pole back in the base, thats the trick. Also, if you try to run a stiff pole, make sure to either use pinch bolts or have something to support the lower pole. If you just crank on the pole bolt it will warp and eventually crack the thin aluminum base of the factory pole.
If you think you need it shorter you can pick up a Xmetal -2.5 to -3.5 adjustable turn plate. That way you can try out some different lengths.
My setup is silly short. -3.5 pole, -2.5 steerplate, shortend bracket moved forward 4"... its almost too much.
GL